Antibiotics and painkillers

What you'll learn

This revision guide covers the essential knowledge about antibiotics and painkillers required for AQA GCSE Biology. You'll understand how these medicines work differently, why antibiotics cannot treat viral infections, and the serious issue of antibiotic resistance. This topic appears in the Infection and response section and links to practical work on investigating the action of antibiotics.

Key terms and definitions

Antibiotics — medicines that kill bacteria or inhibit their growth without damaging human body cells; work by targeting bacterial structures or processes that differ from human cells.

Painkillers — medicines that relieve symptoms of disease (such as pain, inflammation or fever) but do not kill pathogens; examples include aspirin, paracetamol and ibuprofen.

Pathogens — microorganisms that cause disease, including bacteria, viruses, fungi and protists.

Antibiotic resistance — when bacteria evolve and develop characteristics that allow them to survive exposure to antibiotics that would normally kill them or stop their growth.

Mutation — a random change in the DNA of an organism that can lead to new characteristics, including resistance to antibiotics.

MRSA — Methicillin-resistant Staphylococcus aureus, a strain of bacteria resistant to most common antibiotics, causing serious infections particularly in hospitals.

Efficacy — the effectiveness of a medicine in treating a disease or condition; measured through clinical trials and testing.

Clinical trials — controlled experiments on humans to test the safety and effectiveness of new medicines before they can be prescribed to patients.

Core concepts

How antibiotics work

Antibiotics are specific medicines that target bacterial pathogens. They work in several ways:

• Inhibiting bacterial cell wall synthesis, causing the bacteria to burst
• Interfering with protein synthesis in bacterial ribosomes
• Disrupting bacterial DNA replication
• Damaging bacterial cell membranes

The key principle is that antibiotics target structures or processes that exist in bacterial cells but not in human cells. This is why antibiotics can kill bacteria without harming the patient's own cells.

Penicillin, discovered by Alexander Fleming in 1928, was the first widely used antibiotic. It works by preventing bacteria from forming cell walls properly, which causes them to rupture and die. Human cells lack cell walls, so penicillin does not damage them.

Antibiotics are used to treat bacterial infections such as:

• Bacterial pneumonia
• Bacterial meningitis
• Tuberculosis (TB)
• Strep throat
• Urinary tract infections (UTIs)
• Infected wounds

Why antibiotics don't work on viruses

This is a crucial concept frequently tested in exams. Antibiotics cannot kill viruses or cure viral infections for several important reasons:

Structural differences: Viruses are not cells. They lack the structures that antibiotics target, such as cell walls, ribosomes, and metabolic pathways. Viruses are simply genetic material (DNA or RNA) wrapped in a protein coat.

Viral reproduction: Viruses reproduce by inserting their genetic material into host cells and using the host cell's machinery to make copies of themselves. Since they rely on human cells for reproduction, any medicine that interfered with this process would also damage human cells.

Size and complexity: Viruses are much smaller and simpler than bacteria. They have no cell wall, no cytoplasm, and no organelles for antibiotics to target.

Common viral infections that cannot be treated with antibiotics include:

• Common cold
• Influenza (flu)
• COVID-19
• Measles
• Chickenpox
• HIV/AIDS

Taking antibiotics for viral infections is ineffective and contributes to the serious problem of antibiotic resistance.

How painkillers work

Painkillers (also called analgesics) work completely differently from antibiotics. They do not kill pathogens at all — instead, they treat the symptoms of disease.

Mechanism of action: Most common painkillers work by:

• Blocking pain signals to the brain
• Reducing inflammation at the site of injury or infection
• Lowering fever by acting on the brain's temperature control centre

Important distinction: Painkillers provide symptomatic relief but do not cure the underlying disease. Your immune system must still fight off the infection.

Examples of common painkillers and their uses:

Aspirin:

• Reduces pain, inflammation and fever
• Works by inhibiting enzymes that produce inflammatory chemicals
• Not suitable for children due to risk of Reye's syndrome

Paracetamol:

• Reduces pain and fever
• Does not reduce inflammation significantly
• Safer for children than aspirin
• Overdose causes serious liver damage

Ibuprofen:

• Reduces pain, inflammation and fever
• Can irritate the stomach lining
• Not suitable for people with stomach ulcers

Painkillers can be used for both bacterial and viral infections because they target symptoms, not the pathogen itself. This is why you can take paracetamol for flu symptoms even though antibiotics wouldn't help.

The development of antibiotic resistance

Antibiotic resistance is one of the most serious threats to modern medicine. It occurs through natural selection:

Step 1: Mutation Random mutations in bacterial DNA occasionally produce bacteria with characteristics that make them resistant to certain antibiotics. This is not caused by the antibiotic — mutations are random.

Step 2: Selection pressure When antibiotics are used, they kill non-resistant bacteria but resistant bacteria survive. The antibiotic creates a selection pressure that favours resistant bacteria.

Step 3: Reproduction Resistant bacteria survive and reproduce rapidly (bacteria can divide every 20 minutes). They pass on the resistance gene to offspring.

Step 4: Spread The population of resistant bacteria grows while non-resistant bacteria are eliminated. Eventually, the entire population may be resistant to that antibiotic.

Consequences of antibiotic resistance:

• Infections become harder to treat
• Surgical procedures become more dangerous
• Longer hospital stays and higher healthcare costs
• Increased mortality from previously treatable infections
• Need for stronger, more toxic antibiotics with worse side effects

Preventing and reducing antibiotic resistance

Multiple strategies are essential to slow the spread of antibiotic resistance:

For doctors and healthcare professionals:

• Only prescribe antibiotics for bacterial infections, not viral ones
• Use diagnostic tests to confirm bacterial infections before prescribing
• Choose narrow-spectrum antibiotics when possible (target specific bacteria)
• Avoid prescribing antibiotics unnecessarily to meet patient demands

For patients:

• Complete the full course of antibiotics even if symptoms improve
• Never share antibiotics with others
• Never save antibiotics for later use
• Don't demand antibiotics from doctors for colds or flu

Completing the course: This is critical. Stopping antibiotics early when you feel better leaves the strongest bacteria alive. These are the ones most likely to be partially resistant. Completing the full course ensures all bacteria are killed, including any with partial resistance.

In agriculture:

• Reduce routine use of antibiotics in farm animals
• Stop using antibiotics as growth promoters in livestock
• Improve hygiene on farms to prevent infections naturally

Research and development:

• Develop new antibiotics to replace those that have become ineffective
• Research alternative treatments such as bacteriophages (viruses that infect bacteria)
• Improve rapid diagnostic tests to identify infections quickly

Testing antibiotics in the laboratory

You may need to describe practical work investigating the effectiveness of antibiotics. The standard method uses agar plates:

Method:

1. Sterilise an agar plate containing nutrient agar jelly
2. Spread bacteria evenly across the surface using a sterile spreader
3. Place paper discs soaked in different antibiotics onto the agar
4. Include a control disc soaked in sterile water
5. Seal the plate with tape (not completely sealed to allow oxygen in)
6. Incubate at 25°C for 48 hours (lower than body temperature to prevent growth of human pathogens)
7. Measure the diameter of clear zones (inhibition zones) around each disc

Results interpretation:

• Large clear zone = antibiotic is effective against these bacteria
• Small or no clear zone = bacteria are resistant to this antibiotic
• The clear zone is where bacteria have been killed or growth has been inhibited

Safety and aseptic technique:

• Sterilise all equipment to prevent contamination
• Flame the neck of bottles containing bacteria
• Seal plates to prevent contamination but allow gas exchange
• Incubate at 25°C, not 37°C (human body temperature would culture dangerous pathogens)
• Dispose of cultures safely using autoclave or disinfectant

Worked examples

Example 1: Explaining why antibiotics don't work on viruses (3 marks)

Question: Explain why antibiotics are effective against bacterial infections but cannot treat viral infections.

Mark scheme answer:

• Antibiotics target bacterial structures such as cell walls or ribosomes (1 mark)
• Viruses do not have these structures / viruses are not cells (1 mark)
• Viruses reproduce inside host cells using host cell machinery, so targeting them would damage human cells (1 mark)

Examiner note: You must give specific reasons related to structural or functional differences. Vague answers like "antibiotics only work on bacteria" receive no marks.

Example 2: Natural selection and antibiotic resistance (4 marks)

Question: Describe how bacteria can become resistant to antibiotics through natural selection.

Mark scheme answer:

• Random mutation in bacterial DNA produces a bacterium with resistance to the antibiotic (1 mark)
• When antibiotics are used, non-resistant bacteria are killed but resistant bacteria survive (1 mark)
• Resistant bacteria reproduce and pass on the resistance gene to offspring (1 mark)
• The population becomes mostly or entirely resistant to that antibiotic (1 mark)

Examiner note: Make sure you mention that mutations are random, not caused by antibiotics. Also include the key stages: variation, selection, reproduction, and change in population.

Example 3: Comparing antibiotics and painkillers (4 marks)

Question: Compare how antibiotics and painkillers work to treat a person with a bacterial infection.

Mark scheme answer:

• Antibiotics kill bacteria or prevent their reproduction (1 mark)
• Antibiotics target structures unique to bacteria such as cell walls (1 mark)
• Painkillers relieve symptoms such as pain, fever or inflammation (1 mark)
• Painkillers do not kill pathogens / only the immune system and antibiotics cure the infection (1 mark)

Examiner note: "Compare" questions require you to make clear differences. Use comparative language: "whereas", "but", "in contrast".

Common mistakes and how to avoid them

• Saying antibiotics "kill viruses": This is completely incorrect. Antibiotics have no effect on viruses because viruses lack the cellular structures antibiotics target. Be precise: antibiotics kill bacteria only.

• Confusing painkillers with antibiotics: Remember that painkillers treat symptoms (pain, inflammation, fever) but do not kill pathogens. Antibiotics kill bacteria. A person with flu can take painkillers for symptom relief, but antibiotics won't help because flu is viral.

• Claiming antibiotics cause resistance mutations: Mutations are random and occur naturally. Antibiotics don't cause mutations — they create selection pressure that allows existing resistant bacteria to survive and reproduce.

• Not completing antibiotic courses: Students often write that stopping antibiotics early is fine once symptoms improve. This is dangerous because it leaves the strongest, most resistant bacteria alive. Always complete the full course.

• Vague exam answers: Writing "antibiotics work on bacteria" without explaining how they work or why they're specific to bacteria earns minimal marks. Include mechanism (e.g., "damage cell walls") and specificity (e.g., "human cells lack cell walls").

• Forgetting that painkillers work on both bacterial and viral infections: Since painkillers treat symptoms rather than killing pathogens, they provide relief regardless of whether the infection is bacterial or viral.

Exam technique for "Antibiotics and painkillers"

• "Explain" questions: These require reasons or mechanisms. For antibiotic resistance questions, include all four stages: mutation, selection, reproduction, and population change. For "why don't antibiotics work on viruses", mention structural differences and that viruses reproduce inside host cells.

• Command word "describe": Give factual points without detailed explanation. For antibiotic testing practicals, state the steps in sequence. For describing antibiotic resistance, state what happens without explaining why.

• Extended response questions: Structure answers logically with clear stages. Use scientific terminology correctly: mutation, selection pressure, resistant, gene, reproduce. Link points with connectives: therefore, as a result, consequently.

• 6-mark questions: Plan your answer before writing. Include an introduction stating the key point, then develop 4-5 detailed points with scientific terminology, and conclude. Quality of written communication is assessed, so use proper sentences and check spelling of key terms.

Quick revision summary

Antibiotics kill bacteria or inhibit their growth by targeting bacterial-specific structures like cell walls. They cannot treat viral infections because viruses lack these structures and reproduce inside host cells. Painkillers relieve symptoms such as pain and fever but do not kill pathogens. Antibiotic resistance develops through natural selection when random mutations produce resistant bacteria that survive antibiotic treatment, reproduce, and spread. Prevent resistance by completing antibiotic courses, only using antibiotics for bacterial infections, and avoiding unnecessary prescriptions.